1. Field of the Invention
This invention relates generally to gripping devices for sheet material, such as plastic film, and more particularly to an improved sheet gripping assembly of the general type disclosed in my prior U.S. Pat. No. 4,103,401.
2. Prior Art
Sheet gripping devices of the type to which this invention pertains are useful for a variety of sheet gripping applications. This invention, however, like that described in my above-mentioned patent, is intended primarily for use in constructing greenhouse of the class having a frame structure covered by thin plastic film or sheet material. For this reason, the invention will be described in this particular context.
The type of greenhouse construction referred to involves the assembly of pipes, tubes, wooden struts, or the like, into an open frame structure which is then covered with plastic film or sheet material which forms a weather and wind resistant covering or skin that is transparent to the sun's ultraviolet radiation. The plastic sheet material is attached to the frame structure by sheet gripping devices which are secured to the frame member and firmly grip the sheet material.
A variety of sheet gripping devices for this purpose have been devised. U.S. Pat. No. 3,803,671 discloses a sheet gripping device for this purpose. Simply stated, this patented sheet gripping device comprises an elongate channel shaped base having a longitudinal side opening bounded by longitudinal sheet gripping edges and a sheet gripping bar insertable laterally into the base through its open side. The major width of this gripping bar is greater than the width of the base side opening and either or both the base and the bar are sufficiently resilient to permit the bar to be forced laterally between the sheet gripping edges of the base into the interior channel or cavity within the base.
In use, the base of the sheet gripping device is firmly attached to the frame structure of the greenhouse. A plastic sheet is then placed across the base and tucked into its open side to form a fold within the base. Thereafter, the sheet gripping bar is inserted laterally through the open side of the base to a position within this fold. When the sheet gripping device and plastic sheet are thus assembled, the sheet extends into the interior of the base through its open side, between the gripping bar and one gripping edge of the base, then around the bar, and finally back through the open side of the base, between the gripping bar and the other gripping edge of the base. Accordingly, edgewise tension in the plastic sheet tends to force the sheet gripping bar laterally against the sheet gripping edges on the base, whereby the plastic sheet is firmly gripped between these gripping edges and the gripping bar.
Sheet gripping devices of this general type have one inherent defect which this invention seeks to overcome. This defect resides in the fact that tension in the sheet material not only urges the gripping bar laterally outward against the base gripping edges to grip the sheet material between the bar and gripping edges but also tends to force the bar outwardly between these gripping edges and thereby totally dislodge the bar from the base. If this occurs, of course, the plastic sheet is no longer gripped and is thus free to pull from the base. Tension force in the plastic sheet capable of dislodging the gripping bar from the base may result from wind loads on the sheet or shrinking of the sheet in cold weather. Moreover, in some cases, the plastic sheet material has a double wall thickness with entrapped air between the two walls. Thermal expansion of this air in hot weather creates tension forces in the sheet material which tend to dislodge the gripping bar from the base.
Various procedures have been devised in an attempt to solve this particular problem. My earlier mentioned prior U.S. Pat. No. 4,103,401, for example, discloses one solution to the problem. This latter sheet gripping device or assembly has an elongate channel shaped base with a longitudinal side opening bounded by longitudinal gripping edges, an elongate gripping bar insertable laterally through the side opening into the base, and an additional gripping channel which is also insertable into the base through its open side and itself has an open side bounded by longitudinal gripping edges. When assembled on a greenhouse frame structure to secure a plastic sheet to the structure, the gripping bar and channel are positioned within the base which is attached to the frame structure, and the gripping channel fits about the innermost side of the bar which projects into the channel through its open side. The plastic sheet extends into the base through its open side, between one base gripping edge, on the one hand, and the gripping bar and one gripping edge of the inner gripping channel, on the other hand, then about the innermost side of the gripping bar and channel, and finally back through the side opening of the base between the other base gripping edge, on the one hand, and the gripping bar and the other channel gripping edge, on the other hand.
In this latter sheet gripping device, tension in the plastic sheet urges both the inner gripping bar and inner gripping channel outwardly toward the side opening of the base to grip the sheet material between the gripping bar and the base gripping edges, as in the sheet gripping device of the above mentioned U.S. Pat. No. 3,803,671. In the improved gripping device of U.S. Pat. No. 4,103,401, however, increasing tension in the plastic sheet eventually urges the gripping bar and channel outwardly sufficiently to abut the channel gripping edges against the base gripping edges. The gripping bar, which projects into the open side of the gripping channel, resists lateral deflection of the channel gripping edges toward one another, whereby abutment of the latter edges with the base gripping edges affects gripping of the plastic sheet between these abutting edges and prevents complete dislodgement of the gripping bar from the base.